This invention relates to a cooling device of steam turbines, and more particularly, to a cooling device of a steam turbine of the type suitable for use with a turbine plant of superhigh temperature and pressure.
With a rise in the price of oil as a fuel, a program has been under way all over the world for once again using coal as a fuel. The present tendency of generating plants is to switch from oil to coal as a source of fuel supply. However, the coal-burning fuel power plant suffers the disadvantage that it is lower in overall efficiency than the oil-burning power plant because the rate of auxiliary facilities necessary for carrying out pretreatment of coal and removal of dust from the coal is relatively high. In view of this situation, studies are being conducted on measures for improving the power generating efficiency of coal-burning power plants. To this end, it is known to improve the conditions of steam at the inlet of a steam turbine or to raise the temperature and pressure of the steam. It is known that after the steam at the inlet of a steam turbine attained a pressure of 246 kg/cm.sup.2 and a temperature of 566.degree. C., no rise in pressure and temperature has been achieved. This is attributed in part to the fact that the critical temperature of heat resisting ferrite steel accounting for the majority of materials for producing parts of the plants lies in the vicinity of 560.degree. C. However, proposals have been made, as a result of advances made in the progress of technology on heat resisting materials in, to provide turbine plants of superhigh temperature and pressure by using heat resisting austenite steel material so as to greatly improve the heat cycle efficiency of a steam turbine by raising the temperatue and pressure of the steam at the inlet of the steam turbine to high levels.
Some disadvantages are associated, however, with this heat resisting austenite steel material. One of them is that the higher the high temperature strength of the material, the lower become workability and weldability thereof. This is one of the reasons for an increase in the cost of steam plants. Moreover, steam plants of superhigh temperature and pressure pose a problem in that difficulties are encountered in structural design with regard to relieving thermal stresses and accommodating differences in elongation between various structural components, such as turbine casing and piping, because the steam becomes high in pressure when it becomes high in temperature.
An article entitled "First Commercial Supercritical-Pressure Steam turbine" by C. W. Elston et al. appearing in ASME paper, 55A-159, 1955, shows the use of heat resisting austenite steel material for producing a turbine casing which is cooled by steam of low temperature and high pressure obtained by cooling with jet streams of water a portion of the main steam that is branched from the main steam circuit.
Steam plants of high temperature and steam of the prior art described above have suffered the disadvantage of the plant as a whole is low in efficiency because the main steam of high temperature and pressure has its temperature reduced by means of a temperature reducer. The reduction in efficiency is particularly marked when attempts are made to relieve thermal stresses developing in turbine casing, piping and other structural parts because a large amount of cooling steam must be supplied to accomplish the object of cooling.